12.2.3.3 Anthropogenic forcing

In the SAR (Santer et al., 1996c), pattern-based detection studies took into
account changes in well-mixed greenhouse gases (often represented by an equivalent
increase in CO2), the direct effect of sulphate aerosols (usually
represented by a seasonally constant change in surface albedo) and the influence
of changes in strato-spheric ozone. Recent studies have also included the effect
of increases in tropospheric ozone and a representation of the indirect effect
of sulphate aerosols on cloud albedo. Many models now include the individual
greenhouse gases (as opposed to a CO2 equivalent) and include an
interactive sulphur cycle and an explicit treatment of scattering by aerosols
(as opposed to using prescribed changes in surface albedo). Note that representation
of the sulphur cycle in climate models is not as detailed as in the offline
sulphur cycle models reported in Chapter 5. Detection
and attribution studies to date have not taken into account other forcing agents
discussed in Chapter 6, including biogenic aerosols, black
carbon, mineral dust and changes in land use. Estimates of the spatial and temporal
variation of these factors have not been available long enough to have been
included in model simulations suitable for detection studies. In general, the
neglected forcings are estimated to be small globally and there may be a large
degree of cancellation in their global mean effect (see Chapter
6, Figure 6.8). It is less clear that the individual
forcings will cancel regionally. As discussed in Section 12.4,
this will add further uncertainty in the attribution of the response to individual
forcing agents, although we believe it is unlikely to affect our conclusions
about the effects of increases in well-mixed greenhouse gases on very large
spatial scales.

Global mean anthropogenic forcing
The largest and most certain change in radiative forcing since the pre-industrial
period is an increase of about 2.3 Wm-2 due to an increase in well-mixed
greenhouse gases (Chapter 6, Figure
6.8 and Table 6.1). Radiative forcing here is
taken to be the net downward radiative flux at the tropopause (see Chapter
6). Smaller, less certain contributions have come from increases in tropospheric
ozone (about 0.3 Wm-2), the direct effect of increases in sulphate
aerosols (about -0.4 Wm-2) and decreases in stratospheric ozone (about
-0.2 Wm-2). There is a very uncertain and possibly large negative
contribution from the indirect effects of aerosols. Other factors such as that
due to increases in fossil fuel organic carbon, aviation, changes in land use
and mineral dust are very poorly known and not yet incorporated into simulations
used in formal detection studies. Their contribution is generally believed to
be small relative to well-mixed greenhouse gases, though they could be of importance
on regional scales.

In order to assess temperature changes over the last two decades, Hansen et
al. (1997b) estimated the net radiative forcing due to changes in greenhouse
gases (including ozone), solar variations and stratospheric aerosols from 1979
to 1995 from the best available measurements of the forcing agents. The negative
forcing due to volcanoes and decreases in stratospheric ozone compensated for
a substantial fraction of the increase in greenhouse gas forcing in this period
(see Chapter 6, Table 6.13).

Patterns of anthropogenic forcing
Many of the new detection studies take into account the spatial variation of
climate response, which will depend to some extent on the pattern of forcing
(see also Section 12.2.3). The patterns of forcing vary
considerably (see Chapter 6, Figure
6.7). The magnitude of the overall forcing due to increases in well-mixed
greenhouse gases varies from almost 3 Wm-2 in the sub-tropics to
about 1 Wm-2 around the poles. The warming due to increases in tropospheric
ozone is mainly in the tropics and northern sub-tropics. Decreases in stratospheric
ozone observed over the last couple of decades have produced negative forcing
of up to about 0.5 Wm-2 around Antarctica. The direct effect of sulphate
aerosols predominates in the Northern Hemisphere industrial regions where the
negative forcing may exceed 2 Wm-2 locally.

Temporal variations in forcing
Some of the new detection studies take into account the temporal as well as
spatial variations in climate response (see Section 12.4.3.3).
Hence the temporal variation of forcing is also important. The forcing due to
well-mixed greenhouse gases (and tropospheric ozone) has increased slowly in
the first half of the century, and much more rapidly in recent decades (Chapter
6, Figure 6.8). Contributions from other factors
are smaller and more uncertain. Sulphur emissions increased steadily until World
War I, then levelled off, and increased more rapidly in the 1950s, though not
as fast as greenhouse gas emissions. This is reflected in estimates of the direct
radiative effect of increases in sulphate aerosols. Given the almost monotonic
increase in greenhouse gas forcing in recent decades, this means the ratio of
sulphate to greenhouse gas forcing has probably been decreasing since about
1960 (see Chapter 6, Figure 6.8).
This should be borne in mind when considering studies that attempt to detect
a response to sulphate aerosols. The decreases in stratospheric ozone have been
confined to the last two to three decades.

Uncertainties in aerosol forcing
Some recent studies have incorporated the indirect effect of increases in tropospheric
aerosols. This is very poorly understood (see Chapter 6),
but contributes a negative forcing which could be negligible or exceed 2 Wm-2.
The upper limit would imply very little change in net global mean anthropogenic
forcing over the last century although there would still be a quite strong spatial
pattern of heating and cooling which may be incompatible with recent observed
changes (see, for example, Mitchell et al., 1995a). A negligible indirect sulphate
effect would imply a large increase in anthropogenic forcing in the last few
decades. There is also a large range in the inter-hemispheric asymmetry in the
different estimates of forcing (see Chapter 6, Table
6.4). Given this high level of uncertainty, studies using simulations including
estimates of indirect sulphate forcing should be regarded as preliminary.

Summary
Well-mixed greenhouse gases make the largest and best-known contribution to
changes in radiative forcing over the last century or so. There remains a large
uncertainty in the magnitude and patterns of other factors, particularly those
associated with the indirect effects of sulphate aerosol.